1. Field of the Invention
The present invention generally relates to a combustion chamber for internal combustion engines. More specifically, it involves arrangements for internal combustion engine combustion chambers by which not only high fuel economy and high power output are both obtained by controlling the amount of fuel film to be formed adhering onto the inner wall of the combustion chamber, but also white exhaust fumes and NOx are decreased.
2. Background Art
Among diesel engine designs that attempt to achieve high efficiency combustion by directly injecting fuel into a combustion chamber, there are the so-called MAN-M type engines, shown in FIG. 3, and the multi-jet type diesel engines, shown in FIGS. 4 and 5 of the drawings hereof.
In the MAN-M type engines, as shown in FIG. 3, while swirl (S) is generated in the combustion chamber (b), which is recessed deep in the piston head part (a) forming a spherical cavity, fuel mist (F.sub.1) is injected thereinto from the fuel nozzle (c) so as the adhere to the combustion chamber (b) in the form of fuel film. The wall evaporation rate of the fuel film is then controlled by the swirl (S).
Thus, the MAN-M method promotes mixing of air and fuel and combustion thereof by collecting the fuel as it evaporates in the outside of the swirl (S), and is capable of lowering the combustion temperature, thereby achieving low emissions of NOx and noise. However, this method is unsatisfactory because under cold-starting conditions or when idling the engine at very cold temperatures, large amounts of bluish-white fumes and HC (hydrocarbons) are exhausted, since under such conditions, favorable wall evaporation is not attainable because the temperatures of the combustion chamber (b) wall and the combustion air are both low.
This is because: (1) the combustion chamber is not quickly heated to a temperature that will allow the fuel to evaporate and be ignited and, (2) the fuel mist, which has strong penetration due to the sole nozzle hole, further lowers the combustion chamber wall temperature to greatly limit the ability of the fuel to evaporate. Consequently, the amount of evaporated fuel is insufficient compared to the volume of the combusion chamber, and as the evaporated fuel is stirred by the swirl (S), the concentration of the air-fuel mixture gas (referred to as "mixture gas" hereinafter) within the combustion chamber becomes excessively lean, thereby aggravating combustion.
In the combustion chamber of the multi-jet type diesel engine shown in FIG. 4, on the other hand, nozzle holes (g) of the fuel injection nozzle (c.sub.1) are disposed in the combustion chamber (b.sub.1 ) of piston head part (a) so as to respectively face the inner walls (e), which are arranged so as to divide the circumference of the combustion chamber (b.sub.1) more or less equally.
The arrangement shown in FIG. 4 attempts to attain combustion of high output and low fuel consumption rate by creating a premixed gas having good ignitability and combustibility by uniformly dispersing the fuel mist in the combustion chamber (b.sub.1). However, this method is not entirely satisfactory, in that because the penetration of fuel mist for a given strength of swirl becomes weaker as the number of the nozzle holes increases, most of the fuel mist is mixed with that part of the air which is distributed in the force flow (D) of the swirl (S), namely the air present in the vicinity of the center of the combustion chamber (b.sub.1), resulting in the formation of a rich premixed gas in that locality. This happens because the swirl (S) tends to converge toward center, away from the peripheral walls of combustion chamber (b.sub.1) because of the relative configuration of the diameter of the cylinder (not shown) and the diameter of the opening of the combustion chamber (b.sub.1). Therefore, the combustion mode is such that ignition takes place in the vicinity of the combustion chabmer (b.sub.1) center, from which fire propagates rapidly.
Although this is advantageous in terms of the power output and the fuel economy, a problem still remains in that the rapid rise both in the combustion peak temperature and the cylinder pressure gives rise to an increase in NOx and combustion noise.
In the multi-jet type diesel engine combustion chamber (disclosed in Japanese Utility Model Laid Open No. 41229/82), shown in FIG. 5, a fuel injection nozzle (c.sub.2) is disposed so as to eccentrically face the center of the combustion chamber (b.sub.2) , with its plural nozzle holes (g1) so disposed that each of the nozzle holes (g1) faces an inner wall (e.sub.1) of combustion chamber (b.sub.2), and a low thermal conductivity material (h) is attached to that part of the wall (e.sub.1) which is on the closer side in terms of the distance between the nozzle hole (g1) and the wall (e1).
The arrangement of FIG. 5 described above provides fuel mist to the inner wall of the eccentric side thereof by eccentrically disposing the fuel injection nozzle with regard to the combustion chamber center, and at the same time evaporates that fuel mist by the low thermal conductivity member (h).
Thus, in the arrangement of FIG. 5, premixed gas is formed at both the inner side and the outer of the swirl (S), but a problem exists in that, owing to the eccentrically oriented injection direction, the fuel mist from the nozzle hole located at the opposite side of the eccentricity has to travel longer, resulitng in weakened penetration, so that most of that fuel mist is distributed and vaporized in the inner side of the swirl or the central area of the combustion chamber (b2). This means that richer premixed gas is generated in the vicinity of the combustion chamber center as in the case of the arrangement of FIG. 4.
In yet another proposed arrangement, the combustion chamber inner wall is wholly covered with the aformentioned low thermal conductivity member so as to improve adiabatic efficiency. Such an arrangement is not favored, however, because it creates other problems. For example, the intake air is inordinately heated up due to the improved adiabatic character, and this results in either decreased suction efficiency, or the fuel is untimely ignited before it is gasified to form a proper mixture gas.